This comes as a bit of a blow following the good news last year that the hole in the ozone above Antarctica appears to be healing thanks to a ban on chlorofluorocarbons (CFCs). The average size of the hole, first discovered in 1985, has shrunk from 20 million square kilometers (12.4 million square miles) in 2000 to 16 million square kilometers (9.9 million square miles) in 2017, though it could take decades to heal completely.

"Ozone has been seriously declining globally since the 1980s, but while the banning of CFCs is leading to a recovery at the poles, the same does not appear to be true for the lower latitudes,” Joanna Haigh, Co-Director of the Grantham Institute for Climate Change and the Environment at Imperial College London and study co-author, explained in a statement.

"The potential for harm in lower latitudes may actually be worse than at the poles. The decreases in ozone are less than we saw at the poles before the Montreal Protocol was enacted, but UV radiation is more intense in these regions and more people live there."

So, how widespread is the damage? Thinning appears to be occurring in the lower stratosphere, at 15 to 24 kilometers (9 to 15 miles) altitude, above highly populated regions between 60 degrees north and 60 degrees south. To put that into perspective, Oslo, Norway, is 59.91 degrees north and Cape Horn, Chile, is 55.98 degrees south.

The scientists aren't entirely sure what's causing atmospheric deterioration but suspect it has something to do with climate change shifting the pattern of atmospheric circulation and the release of very short-lived substances (VSLSs) into the Earth's atmosphere. The latter includes chemicals used in solvents, degreasing agents, and paint strippers.

"The finding of declining low-latitude ozone is surprising, since our current best atmospheric circulation models do not predict this effect," William Ball, an atmospheric researcher at ETH Zurich and first author, said in a statement. "Very short-lived substances could be the missing factor in these models."

Previous research has hinted at changes in the Earth's ozone but this appears to be the first to combine datasets since 1985 to show that atmospheric thinning is a long-term trend. Now, the researchers hope to gather more precise data on the decline of the ozone layer and work out the cause.

The good news is that "the decline now observed is far less pronounced than before the Montreal Protocol," added co-author Thomas Peter. "But we have to keep an eye on the ozone layer and its function as a UV filter in the heavily populated mid-latitudes and tropics."